1. Field of the Invention
The invention relates generally to High Electron Mobility Transistors (HEMTs). More particularly, the invention relates to a heterogeneous integration of low noise HEMT amplifiers with Pseudomorphic HEMT (PHEMT) power amplifiers or switches.
2. Description of Related Art
When donors are placed in a material with a higher conduction band energy than the channel, and close to the channel, they create an electron gas with high electron mobility in the channel. Transistors that implement this concept are known in the art as High Electron Mobility Transistors (HEMT).
Most HEMTs require at least two materials: barrier and channel materials. The barrier material that contains the donor must have a higher conduction band energy, but also improves the device breakdown by having a wider bandgap to accommodate high fields. The channel material that makes up the conducting layer is selected based on the transport properties of the electrons, with the bandgap also being a consideration to support high fields and hence high voltages. Typically, a HEMT system is made from Gallium Arsenide (GaAs) with Aluminum Gallium Arsenide (AlGaAs). The donor-containing wide bandgap material is AlGaAs and the conducting channel is GaAs.
HEMTs can be used for Monolithic Microwave Integrated Circuits (MMICs) such as low-noise amplifiers for receivers and power amplifiers for transmitters. There are four main reasons for their resilience. First, parasitic device resistance is small due to the high electron mobility (5,000-35,000 cm2/Vs) and carrier concentrations (about 1×1012 cm−2 to about 5×1012 cm−2) achievable across the various material choices. Second, the electron velocities for the channel materials are high with peak values between about 1×107 cm2/s to about 5×107 cm2/s. Third, the technology has enjoyed relatively simple performance gains through gate-length scaling with 100 nm long gates now in production and down to 25 nm in development. Fourth, for power amplifiers, the HEMTs offer useable breakdown voltages and do not suffer from thermal runaway, which is a weakness of competing bipolar technologies.
High performance MMICs of up to 100 GHz use a Pseudomorphic HEMT (PHEMT) technology. PHEMT devices have a small amount of Indium (In) added to the GaAs channel. The growth of the channel is constrained to a critical thickness, that if exceeded, dislocations nucleate and the device properties degrade, and if maintained below the critical thickness, the channel material remains pseudomorphic with the same in-plane lattice constant as the GaAs host substrate but a larger lattice constant in the growth direction. The advantage of using a PHEMT device is higher conductivity channels with higher electron velocities and improved minimum noise figures. However, the disadvantage of using a PHEMT is a lower breakdown voltage when the concentration of Indium (In) is greater than the amount required for lattice matching to the substrate or barrier.
While prior art transistors can be optimized for either a low-noise amplifier for receivers or a power amplifier for transmitters, they cannot be optimized simultaneously with one another on the same substrate. This is because the low-noise amplifier cannot withstand large voltages used for the power amplifier without sacrificing its low noise characteristics. Separate substrates are typically used when a device, such as a telephone, utilizes a receiver and a transmitter. With an increasing demand for improved transistors, there remains a continuing need in the art for transistor that can simultaneously sustain high power drive and high sensitivity as in a low-noise amplifier.